Department of Physics, Carleton University, Ottawa, Ontario K1S 5B6, Canada.
Med Phys. 2012 Jul;39(7):4365-77. doi: 10.1118/1.4729737.
Model-baseddose calculations (MBDCs) are performed using patient computed tomography (CT) data for patients treated with intraoperative (125)I lung brachytherapy at the Mayo Clinic Rochester. Various metallic artifact correction and tissue assignment schemes are considered and their effects on dose distributions are studied. Dose distributions are compared to those calculated under TG-43 assumptions.
Dose distributions for six patients are calculated using phantoms derived from patient CT data and the EGSnrc user-code BrachyDose. (125)I (GE Healthcare/Oncura model 6711) seeds are fully modeled. Four metallic artifact correction schemes are applied to the CT data phantoms: (1) no correction, (2) a filtered back-projection on a modified virtual sinogram, (3) the reassignment of CT numbers above a threshold in the vicinity of the seeds, and (4) a combination of (2) and (3). Tissue assignment is based on voxel CT number and mass density is assigned using a CT number to mass density calibration. Three tissue assignment schemes with varying levels of detail (20, 11, and 5 tissues) are applied to metallic artifact corrected phantoms. Simulations are also performed under TG-43 assumptions, i.e., seeds in homogeneous water with no interseed attenuation.
Significant dose differences (up to 40% for D(90)) are observed between uncorrected and metallic artifact corrected phantoms. For phantoms created with metallic artifact correction schemes (3) and (4), dose volume metrics are generally in good agreement (less than 2% differences for all patients) although there are significant local dose differences. The application of the three tissue assignment schemes results in differences of up to 8% for D(90); these differences vary between patients. Significant dose differences are seen between fully modeled and TG-43 calculations with TG-43 underestimating the dose (up to 36% in D(90)) for larger volumes containing higher proportions of healthy lung tissue.
Metallic artifact correction is necessary for accurate application of MBDCs for lung brachytherapy; simpler threshold replacement methods may be sufficient for early adopters concerned with clinical dose metrics. Rigorous determination of voxel tissue parameters and tissue assignment is required for accurate dose calculations as different tissue assignment schemes can result in significantly different dose distributions. Significant differences are seen between MBDCs and TG-43 dose distributions with TG-43 underestimating dose in volumes containing healthy lung tissue.
在梅奥诊所罗切斯特分校，使用患者计算机断层扫描 (CT) 数据为接受术中 (125)I 肺近距离放射治疗的患者进行基于模型的剂量计算 (MBDC)。考虑了各种金属伪影校正和组织分配方案，并研究了它们对剂量分布的影响。将剂量分布与根据 TG-43 假设计算的剂量分布进行比较。
使用源自患者 CT 数据的体模和 EGSnrc 用户代码 BrachyDose 计算六名患者的剂量分布。(125)I (GE Healthcare/Oncura 模型 6711) 种子被完全建模。将四种金属伪影校正方案应用于 CT 数据体模：(1) 不校正，(2) 在修改后的虚拟正弦图上进行滤波反投影，(3) 在种子附近对高于阈值的 CT 数进行重新分配，以及 (4) (2) 和 (3) 的组合。组织分配基于体素 CT 数，质量密度使用 CT 数到质量密度校准进行分配。对经过金属伪影校正的体模应用了三种具有不同详细程度的组织分配方案（20、11 和 5 种组织）。还根据 TG-43 假设进行了模拟，即在均匀水中的种子没有种子间衰减。
未校正和金属伪影校正体模之间观察到显著的剂量差异（D(90) 高达 40%）。对于使用金属伪影校正方案 (3) 和 (4) 创建的体模，剂量体积指标通常非常吻合（所有患者的差异小于 2%），尽管存在明显的局部剂量差异。三种组织分配方案的应用导致 D(90) 的差异高达 8%；这些差异在患者之间有所不同。全模型与 TG-43 计算之间存在显著的剂量差异，TG-43 低估了剂量（在 D(90) 中高达 36%），对于包含较高比例健康肺组织的较大体积。
金属伪影校正对于肺近距离放射治疗的 MBDC 准确应用是必要的；对于关注临床剂量指标的早期采用者，更简单的阈值替换方法可能就足够了。为了准确计算剂量，需要严格确定体素组织参数和组织分配，因为不同的组织分配方案可能会导致显著不同的剂量分布。MBDC 与 TG-43 剂量分布之间存在显著差异，TG-43 低估了包含健康肺组织的体积中的剂量。
